1. Field of the Invention
The present invention relates to new isocyanate prepolymers based on selected starting materials, a process for their preparation and their use for producing sealing and coating compositions.
2. Description of the Prior Art
Binders for sealing or coating compositions based on isocyanate prepolymers and compounds which are (potentially) reactive with isocyanates are very well known. For example, DE-OS 1,520,139 describes a process for preparing moisture hardening mixtures of polyisocyanates and polyketimines or polyaldimines, wherein isocyanate prepolymers (NCO prepolymers) are preferably used as polyisocyanates. DE-AS 1,240,654 describes a process for preparing cross-linked plastics from NCO prepolymers and special aromatic diamines. DE-OS 2,018,233 describes compositions made from compounds with isocyanate groups and polyoxazolidines, which can be hardened by moisture.
All of these publications have the common feature that the NCO prepolymers used are those known from polyurethane chemistry, e.g., those prepared by reacting high molecular weight polyhydroxyl compounds, such as polyetherpolyols or polyesterpolyols, with excess amounts of di- or polyisocyanate.
For coating externally located mineral substrates, there is a need for NCO prepolymers which are simultaneously resistant to the effects of light and to saponification. NCO prepolymers based on polyether polyols have a high resistance to saponification but are known to be less stable to light. NCO prepolymers based on polyester polyols, on the other hand, have a high resistance to light but may not be applied in direct contact with mineral substrates due to their poor resistance to saponification.
Although NCO prepolymers based on polycarbonate polyols satisfy the requirements of resistance to light and saponification, these prepolymers are extremely viscous and crystalline and can only be applied by using large amounts of solvent.
Although, prepolymers based on polyester carbonate polyols of the type described in DE-OS 3,200,430 can be used, they have the disadvantage that they are always very viscous so they can only be processed by adding plasticizers or solvents.
The use of solvents, however, is known to be a disadvantage from an ecological standpoint. The use of plasticizers is also associated with disadvantages. Plasticizers, which remain in the coatings, can impair the mechanical properties of polymers, reduce adhesion to the substrate and in the long term migrate out of the coatings.
It is an object of the present invention to provide new single or multi-component binders based on NCO prepolymers, which have lower viscosities than comparable prior art binders, while being highly resistant to light and hydrolysis.
This object may be surprisingly achieved by the isocyanate prepolymers containing ether, ester and carbonate groups described in more detail below, even though these prepolymers have both light-sensitive polyether chains and hydrolysis-sensitive ester groups.